Phthalic anhydride can be obtained from oxidation of o-xylene with air both in the vapor phase in the presence of a vanadium catalyst promoted by hydrogen bromide as disclosed by U.S. Pat. No. 2,954,385 or in the liquid phase in the presence of cobalt, or manganese or cobalt and manganese in combination with a source of bromine as disclosed by British Patent Specification No. 856,245; U.S. Pat. No. 3,402,184; a copending U.S. patent application Ser. No. 867,050 filed Jan. 3, 1978 and Ser. No. 961,763 filed Nov. 17, 1978. The processes of the last patent applications yield a phthalic acid product which is separated from the metal components of catalysis and other high boiling impurities by dehydration to and evaporation of a partially purified anhydride. All of such obtained phthalic anhydride products contain bromine-substituted oxygen-containing compounds and low molecular weight bromine-containing compounds which in general are readily removed by a selective condensation of a vapor-gas mixture in a partial purification step. The bromine and oxygen-containing aromatic compounds are more difficult to remove from partially purified phthalic anhydride because such bromo compounds have physical characteristics; e.g., vapor pressure, rather close to the physical characteristics of the anhydride.
Insofar as we are aware, there is no prior art directed to the problem of removing such bromo-aromatic impurities from phthalic anhydride. It might be thought that said bromo-aromatic impurities would be removed by heating such impure anhydride to and/or above its boiling temperature in the absence or in the presence of alkali metal hydroxide or phthalate according to U.S. Pat. No. 2,670,325 to decrease color and odor formers or according to copending U.S. patent application Ser. No. 898,930 filed Apr. 24, 1978 to decrease phthalide content. However, such heating of the anhydride at from 250.degree. C. up to 280.degree. C. for up to four hours in the presence of alkali metal hydroxides and phthalates do not decrease the anhydride's bromine content below about 30 weight parts per million weight parts of the anhydride. Also heating such anhydride at 250.degree. C. for 2 hours in the presence of debrominators zinc or iron suggested by U.S. Pat. No. 3,642,883 did not decrease the bromine content below 30 ppm.
We provide hereafter demonstrations of the ineffectiveness of the prior art described and suggested processes for purification of phthalic acid, phthalic anhydride and removal of bromine from organic compounds in general.
To better understand the advance of the present inventive contribution for decreasing the bromine content of partially purified phthalic anhydride data are presented concerning the starting impure phthalic anhydride, further purification thereof according to some prior art methods used with phthalic anhydride or with other compounds (e.g., bromine removal from benzoic acid with zinc) and an attempt at bromine removal by only activated carbon. Examples of the use of such prior art described or suggested processes for bromine removal are presented in the Comparative Examples to follow wherein "PAN" is used to designate phthalic acid anhydride.
TABLE I ______________________________________ BROMINE REMOVAL FROM PARTIALLY PURIFIED PAN OF 91 PPM BROMINE CONTENT Heat Treatment PAN Product at 250.degree. C. for of Fractionation Comparative 2 Hours and at 0.13 Atmos. Pressure Example No Addition Agent Reflux Ratio Bromine, ppm ______________________________________ 1 None 1:1 82 2 K-Biphthalate 1:1 66 ______________________________________
TABLE II ______________________________________ BROMINE REMOVAL FROM PARTIALLY PURIFIED PAN Heat Treatment PAN Product at 250.degree. C. for of Fractionation Comparative 2 Hours and at 0.13 Atmos. Pressure Example No. Addition Agent Reflux Ratio Bromine, ppm ______________________________________ 3 None 1:1 67 4 Zinc 1:1 58 5 Pd/C 1:1 32 ______________________________________
TABLE III ______________________________________ BROMINE REMOVAL FROM PARTIALLY PURIFIED PAN Heat Treatment PAN Product at 250.degree. C. for of Fractionation Comparative 2 Hours and at 0.13 Atmos. Pressure Example No. Addition Agent Reflux Ratio Bromine, ppm ______________________________________ 6 None 3:1 63 7 Na-Phthalate 5:1 31 8 K-Phthalimide 5:1 36 9 K-Biphthalate 5:1 44 ______________________________________
The use of sodium and potassium salts (Comparative Examples 2, 7, 8 and 9) as additive for the step of heat treating partially purified phthalic anhydride is not any more effective with respect to bromine removal than the use of Pd/C in such step.
The use of catalytic hydrogenation of similar impure phthalic anhydride or bromo-impurities per se is not, as the following data will demonstrate, effective for removing bromine from partially phthalic anhydride. In TABLE IV to follow, three differently supported palladium catalysts: Pd dispersed on the surface of activated carbon (Pd/C), Pd dispersed on alumina (Pd/Aa) and Pd dispersed on chromia-alumina (Pd/Cra-Aa), are used at a hydrogen partial pressure of 0.1 atmosphere and reflux temperature (275.degree. C.) of partially purified PAN (hereafter "Reflux" Process) or at a hydrogen partial pressure of 6.7 atmospheres and a temperature of 299.degree. C. (hereinafter "299.degree. C." Process). The partially purified, liquid PAN was caused to flow through a bed of the catalyst used. A sample of the partially purified PAN is taken before catalytic hydrogenation (0 hr.), and at 0.5 hr., 1.0 hr., and 2.0 hr. after start of such treatment.
TABLE IV ______________________________________ COMPARATIVE CATALYTIC HYDROGENATION FOR BROMINE REMOVAL ______________________________________ Example PAN's Bromine Content, wt. % No. Product Catalyst Process ______________________________________ 11 PAN Pd/C 299.degree. C. 12 PAN Pd/Cra-Aa Reflux 13 Br-BA* Pd/C 299.degree. C. 14 Br-BA Pd/Cra-Aa 299.degree. C. 15 Br-BA Pd/C Reflux 16 Br-BA Pd/Aa Reflux 17 Br-BA Pd/Cra-Aa Reflux ______________________________________ *"Br-BA" is bromobenzoic acid.
PAN's Bromine Content, wt. % Example No. 0 hr. 0.5 hr. 1.0 hr. 2.0 hr. ______________________________________ 11 0.16 0.18 0.17 0.16 12 0.047 N.S.* 0.043 0.042 13 0.10 0.07 0.05 0.04 14 0.10 0.06 0.05 0.03 15 0.10 0.08 0.07 0.06 16 0.10 0.08 0.07 0.06 17 0.09 0.06 0.06 0.05 ______________________________________ *"N.S." is No Sample analyzed.
The heat treating of partially purified PAN is conducted at temperatures of 200.degree. C., 250.degree. C., and 275.degree. C. for four hours in the presence of 0.28 wt.% KOH as additive or no additive (NONE) and thereafter fractionally distilled at a pressure of 0.13 atmosphere to remove a benzoic acid-containing first fraction and then to remove a PAN product fraction. The bromine content of the feed and of the resulting products are shown below in TABLE V.
TABLE V ______________________________________ Comparative Heat Treatment Bromine Content, ppm Example No. Additive Temp. .degree.C. Feed Product ______________________________________ 18 KOH 200 1074 128 19 NONE 200 1055 116 20 KOH 250 885 68 21 NONE 250 950 80 22 KOH 275 692 30 23 NONE 275 772 38 ______________________________________
We have now discovered a technique for removing the bromine content of phthalic anhydride contributed by the bromine and oxygen-substituted aromatic compounds before or after, preferably before, heating the impure anhydride at or above its boiling point temperature. Such technique is practiced on phthalic anhydride in the vapor phase and can be conveniently conducted with the anhydride vaporized from the dehydration of o-phthalic acid, or vaporized from the heating step generally also used to condense color bodies and color formers to higher boiling products or vaporized in the final purification of the anhydride by fractional distillation.